(a) Field of the Invention
The present invention relates to a shim of cassette tapes and a method of shaping the same.
(B) Description of the Prior Art
Generally, a cassette for tape recorders, as shown in FIGS. 1 and 2, is adapted to receive a tape c having two retainer sheets (shims) a and b in the form of films placed on the front and back sides thereof in a space defined by a cassette body d and a lid e.
Conventionally, said retainer sheets a and b have been made of Teflon or special paper coated with a synthetic resin material. However, the conventional retainer sheets a and b made of such materials are expensive since the material cost is high. Further, the rotation of the tape c is attended with the buildup of static electricity. The sheet material is lacking in rigidity, so that the sheets tend to bend or wrinkle, making the assembling operation inefficient.
Further, the retainer sheet (shim) for use with the cassette is generally made of a self-lubricating material and has a shape as shown in FIG. 3, punched at the center and at opposite sides of a sheet body 1 to provide a peep window 2 for watching the winding condition of the tape and two shaft apertures 3 and 4 for winding and rewinding the tape, the four corneres of the sheet body being cut away. Thus, the shim for the cassette is a sheet of synthetic resin having a thickness of the order of 0.1 mm to provide aganist the rubbing between the cassette body, tape and tape reels disposed on both sides. Conventionally, such sheet has been made of a relatively soft material, e.g., tetrafluoroethylene. However, since the material is soft as described above, the bending would be very severe adjacent the peep window unless the region above the peep window is uninterrupted, such severe bending causing inconvenience to the assembling of the cassette.